The continuous extrusion of plastic net started in about 1956 in accordance with the process of the Mercer U.S. Pat. No. 2,919,467. The process is carried out by means of two rotating die members. In essence, there is an outer rotating die member in the form of a flat horizontal circular plate having an opening in the center thereof and an inner rotating die member in the form of a flat circular plate which is nested in the opening of the outer die member. Sliding contact between the two die members establishes a liquid seal. Each die member has a plurality of spaced orifices in the form of open grooves in the contacting surfaces between the two die members. Strands of plastic are continuously extruded in vertical direction in a circle through the open groove orifices and the die members are preferably counter rotated so that each time an open groove orifice in the outer rotating die member is aligned with an open groove orifice in the inner die member an integral joint is formed between adjacent strands. The resulting product is an extruded tube of plastic net which in general has diamond shaped openings extending along the length of the tube. The Mercer process has been and now is widely used and there are licensees using the process in all of the more important industrial countries in the world.
The United States and French licensees of the Mercer process each independently made a modification wherein the outer die is held stationary while the inner die member is reciprocated in vertical direction to separate the contacting surfaces between the two die members. In the preferred structure, there are no open groove orifices in the contacting surface of one of the die members. A plurality of spaced plastic strands are continuously extruded vertically in a circle through the open groove spaced orifices in the stationary outer die member. The inner die member is periodically separated from the outer die member to separate the contacting surfaces and thereby extrude a transverse circular strand that forms an integral joint that connects the transverse circular strand with all of the spaced vertical strands. As a result a tube of plastic net is extruded which in general has square shaped openings extending along the length of the tube. This continuous extrusion of square mesh net is described in the Hureau U.S. Pat. No. 3,252,181 and in the Galt U.S. Pat. No. 3,384,692 and in the Lemelson Reissue U.S. Pat. No. RE 28,600. Square mesh net is also being extruded commercially in all of the more important industrial countries throughout the world.
There are also a number of other modified forms of the basic Mercer process in use. In one form plastic strands are continuously extruded through a plurality of spaced orifice openings arranged in a straight line in a stationary die member. There are a plurality of spaced cooperating nozzles having orifices therein which are moved back and forth between two adjacent orifices in the stationary die member. Plastic strands are continuously extruded through the nozzle orifices and each time the nozzle orifice contacts an orifice in the stationary die the two strands are welded together and as the nozzle moves away, the strands separate until the nozzle orifice contacts the next adjacent orifice in the stationary die member when these two strands are welded together. As a result a plastic net is formed with a diamond type opening along the length of the extruded net. The net may be extruded as a flat sheet or as a tube by using a circular stationary die member. This modified form of the basic Mercer process is described in the Nalle U.S. Pat. No. 3,127,298. It is not necessary to use the Nalle nozzle orifices. For example, a plurality of spaced open groove orifices may be arranged in the contacting surfaces of each of two flat plate die members. At least one of the plates is adapted to slide back and forth so that any one groove orifice in the moving plate will make contact with and move between two adjacent groove orifices in the stationary plate while plastic strands are continuously extruded through the orifices. As a result, the strand from any one groove orifice in the moving plate continuously forms connecting links between two adjacent strands extruded by the stationary plate to form a net structure. This modified form of apparatus is shown in FIG. 8 of the Mercer U.S. Pat. No. 2,919,467. Instead of sliding the moving plate, it may be periodically displaced relative to the stationary plate to separate the two contacting surfaces. In this case, the contacting surface of the moving plate may be smooth without any groove orifices therein. When the surface of the plates are in contact a first plurality of spaced strands will be extruded from the stationary plate and when the contacting surfaces are periodically separated a transverse strand will be extruded to connect all of the first plurality of spaced strands into a net structure with integral joints. This modification is shown in FIG. 21 of the Hureau U.S. Pat. No. 3,252,181.
In another modified form of apparatus, plastic is continuously extruded through an open slit orifice which can be arranged in a circle or straight line in a first stationary die member. A second cooperating moveable die member is employed to stop the flow of plastic at one or more spaced areas along the fixed orifice opening in the stationary die member. In the most simple form of structure, the second moveable die member is in the form of a comb that slides back and forth across the orifice slit in the stationary die member. When the teeth of the comb are retracted from the slit a continuous strand of plastic is extruded and when the teeth of the comb interrupt the slit a plurality of spaced strands are extruded transverse to the continuous strand of the slit. All of the strands are integrally joined together in the resulting net structure. This modification is shown in FIG. 1 of the Hureau U.S. Pat. No. 3,252,181. The same principal may be employed in extruding a tube of net from an annular slit orifice as illustrated in FIG. 12 of the Hureau U.S. Pat. No. 3,252,181. In the place of a comb, the moveable die member may be in the form of a flat plate having an open slit therein in the form of a comb or any other desired configuration. The slit in the moving plate periodically interrupts the fixed slit opening in the stationary die member to form a net structure. This modification is shown in FIG. 10 of the Hureau U.S. Pat. No. 3,252,181.
The disclosures of the above-mentioned issued patent documents are incorporated by reference into the present specification.
In all of these processes for extruding plastic net, at least one set of strands is extruded through a plurality of spaced individual orifices. The second set of strands in the net structure may be extruded through a second set of spaced individual orifices or the second set of strands may be extruded periodically through a continuous orifice slit arranged in a straight line or in a circle. In all cases the two sets of strands are extruded so that the individual strands intersect at an angle and form integral joints in the extruded plastic net. The resulting extruded flat sheet or tube of plastic net is cooled to set the plastic in the strands as for example in a water bath and the net is drawn away from the extrusion orifices by nip rolls or other suitable drawing means. When a tube of net is extruded, it is usually drawn down over a cylindrical mandrel which may stretch the strands and enlarge the openings in the net structure. Stretching the strands over the mandrel partially orients the plastic but in practice the net is characterized as being unoriented.
For many applications, it is desirable to further stretch the net strands and more fully orient the plastic and this may be done by so-called rope orientation where as in the case of net with diamond shaped holes the tube of extruded net is heated and stretched longitudinally to further elongate and orient the strands. Stretching the tube causes it to collapse while the tube is stretched out in rope form. Square mesh net is preferably oriented to advantage in a two step process as described in my copending application Ser. No. 653,541. Here, a flat sheet of extruded net is heated and one set of strands is further stretched and oriented in one direction and in a second separate step the second set of strands are stretched to orient the strands in a second direction. Some plastic net may be oriented at room temperature but as a practical matter the net is heated to speed up and facilitate orientation of the net.
In general extruded plastic net is classified according to the strand count and weight. The strand count is the number of strands per linear inch in each set of strands. The strand count in a net with square or diamond shaped holes may be readily determined by counting the number of holes and fractions thereof per inch of net. The measurement is made at a right angle to one set of strands starting at the center line of a selected strand and the number of holes and fractions thereof in one inch is recorded. A second measurement is made in the same way at a right angle to the second set of strands and recorded. Thus, if there are 4.5 holes per inch of net for one set of strands and 5.5 holes for the second set of strands in one inch of net structure, there will be 4.5 strands per inch for the one set of strands and 5.5 strands per inch for the second set of strands. For convenience this net is characterized as a 4.5.times.5.5 or 5.5.times.4.5 strand count net. A 6.times.6 strand count net has 6 strands (holes) per inch in each set of strands in each of two directions. The expression 4.times.5 strand count net or 6.times.6 strand count net etc. as used in the specification and claims is intended to mean the number of strands per inch in each of the two sets of strands per square inch of net. Weight of the net is usually specified in pounds per one thousand square feet of net.
Plastic net is also made from an extruded flat sheet of plastic by slitting, cutting, perforating or embossing the sheet in a predetermined manner to provide a multitude of small spaced openings of selected geometric configuration. In the embossing process, a multitude of small non-connecting depressions are formed with only a very thin membrane of plastic at the bottom of the depressions which may extend all or only part way across the bottom of each depression. The net is formed by stretching the sheet usually in two different directions as for example in a longitudinal direction and in a transverse direction at a right angle thereto. The stretching in two directions may be carried out simultaneously or in two separate sequential steps. Stretching the sheet causes the embossed membranes to split and fibrillate and it otherwise enlarges and expands the openings with corresponding elongation and attenuation of the full thickness portions of the sheet. Plastic net is produced from an extruded solid sheet of plastic by a number of companies in the United States and the processes are described in British Pat. No. 1,075,487 (U.S. Pat. No. 3,441,638) issued in 1967 to T. J. Smith and Nephew Limited which is incorporated by reference herein. Also see British Pat. No. 982,036 issued Feb. 3, 1965 to FMC Corporation and U.S. Pat. No. 3,881,381 issued May 6, 1975 to Johnson and Johnson and U.S. Pat. No. 3,666,609 issued May 30, 1972 to Johnson and Johnson.